1. Field of the Invention
The present invention relates to intake air control valves used for controlling the flow of intake air that is supplied to an internal combustion engine. In this specification, the term “intake air control valves” is used to broadly include various types of valves and devices for controlling the flow of intake air, such as those known as “throttle bodies”, “throttle valve devices”, “throttle control devices”, “intake air control devices” and “metering valve devices”.
2.Description of the Related Art
A known intake air control valve is disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 6-14635. FIG. 27 shows such a known intake air control valve in a cross sectional view.
According to the intake air control valve shown in FIG. 27, a valve shaft 3 is rotatably supported within a valve body 5. The valve body 5 defines a bore 6, through which intake air flows. A butterfly-type valve plate 1 is inserted into a slit 4 formed in the valve shaft 3 and is fastened thereto by screws 8. The valve plate 1 can open and close the bore 6 of the valve body 5 as the valve shaft 3 rotates. A mount seat 1a protrudes from the valve plate 1 and contacts with the valve shaft 3 in order to prevent the valve shaft 3 from being warped by the tightening force that is applied during the tightening operation of the screws 8.
With the intake air control valve shown in FIG. 27, the valve plate 1 has an elliptical configuration and has a uniform thickness, except for a portion having the mount seat 1a. The elliptical configuration is determined such that an outer diameter of the valve plate 1 as viewed in a direction of flow of air within the bore 6 is slightly smaller than an inner diameter of the bore 6 (hereinafter called “bore diameter”) when the valve plate 1 is in a fully closed position. Therefore, when the valve plate 1 is in the fully closed position, for example, during the idle driving operation, a substantial clearance is produced between the outer diameter of the valve plate 1 and the inner wall of the bore 6. For this reason, the intake air leaks from the upstream side to the downstream side through such a clearance. As a result, the amount of leakage of intake air (hereinafter also called “air leakage amount”) increases.
Even in the case that a usual valve plate having a perfect circular configuration is used, the outer diameter of the valve plate is generally determined to be smaller than the bore diameter. Therefore, also in this case, the intake air flows though the clearance between the outer diameter of the valve plate and the inner wall of the bore when the valve plate is in a fully closed position. As a result, a problem of leakage of the intake air is still caused. Simply configuring the valve plate to have an outer diameter equal to the bore diameter may reduce the air leakage amount. However, with this configuration, the valve plate may tend to frictionally contact with the inner wall of the bore. Therefore, this design is not appropriate, because the operability of the valve plate is degraded.